1. Field of the Invention
The present invention relates to a dot print position adjusting method in a dot matrix printing and to a printing system using this method. More specifically, the invention relates to a print position adjustment to align positions of dots formed by a forward scan and a backward scan in a bidirectional printing and to a print position adjustment to align positions of dots formed by a plurality of print heads.
2. Description of the Related Art
Relatively inexpensive personal computers, word processors and other office equipment have proliferated in recent years. Under these circumstances, printing apparatus to print information from these devices and technologies to increase a printing speed and enhance an image quality of the printing apparatus are being developed one after another. Among the printing apparatus, a serial printer using the dot matrix printing method has attracted attention as a printer that can realize a fast or high quality printing at low cost.
In a printing apparatus that performs a bidirectional printing for an increased speed, if positions on a print medium of dots formed by a forward scan and of dots formed by a backward scan deviate from each other, an image degradation such as line misalignment occurs. That is, when vertical lines perpendicular to the scan direction of the print head are formed by forward scans and backward scans alternately, the positions of dots formed by the forward and backward scans may fail to align, resulting in vertical ruled lines with low straightness. This line misalignment is one of the most popular image impairments observed by the user. Ruled lines are often printed in black and thus the line misalignment tends to be recognized as a problem encountered in black images. The similar problem, however, also occurs with color images.
Such a print position misalignment between the forward and backward scans produces another image impairment called “texture” during a multipass printing which is performed to enhance a print quality. In the multipass printing, print data that can be printed in one scan of print head is masked using a predetermined culling or thinning pattern. In one and the same print area on the print medium an image is formed in two or more scans using a plurality of culling patterns that are complementary to one another. Thus a phenomenon such as the aforementioned line misalignment is unlikely to be observed. However, if a culling pattern used in the forward scan and a culling pattern used in the backward scan deviate from each other, a resulting image will become ununiform. This ununiform image appears in a cycle that depends on the applied mask pattern, so when the entire image is looked at, an unpleasant pattern or texture shows over the entire image. This texture tends to become particularly noticeable in half tone areas of the image printed at high density and high contrast, as when the image is printed in monochrome or on coated paper.
Further, in a printing apparatus with a plurality of print heads, such as yellow, magenta, cyan and black color heads, for example, if dot landing positions from the four print heads deviate from each other, a phenomenon called “color deviation” occurs on the printed image. The color deviation will be briefly explained as follows.
When a blue color is to be formed, a magenta ink and a cyan ink are used. An area on a print medium where dots of the two colors overlap and an area where they do not, produce slightly different colors. In a uniform blue color image, an area with a slightly differing color, if it is small, does not show in the image. But if the dot deviation between the magenta and cyan print heads occurs only during a particular scan, only the areas printed by that scan show up their color difference in the form of bands, resulting in an uneven blue color image. This phenomenon is referred to as “color deviation” in this specification. The “color deviation” does not show so much on plain paper, but on print mediums that produce highly saturated colors, such as coated paper, tends to become more noticeable.
When different colors are printed at adjoining positions by a plurality of print heads, if a deviation occurs between the different colors, an unprinted area or gap is formed at the deviated dot portions, allowing the color of the print medium to be exposed. Since print mediums are mostly white, this phenomenon is called “white blanking.” This phenomenon is particularly noticeable on an image with a strong contrast. For example, in a black image on a color background, if there is an unprinted white area between the black area and the color area, the blank area clearly shows up because of the strong contrast between white and black.
To minimize the above image impairments, many printing apparatus on the market adopt dot adjust value calculation processing. The dot adjust value calculation processing in this specification means processing which—in a printing apparatus forming an image by two printing operations with different printing conditions, such as a first printing during a forward scan and a second printing during a backward scan—calculates an adjust value for aligning the print positions of the first and second printing. The adjust value acquired by the dot adjust value calculation processing denotes such a correction value to adjust timings at which the print head ejects ink during the forward and backward scans in order to align the print positions of the forward scan and the backward scan in a bidirectional printing.
A general procedure of the dot adjust value calculation processing will be explained in the following by taking a bidirectional printing as an example. First, the printing apparatus prints on a print medium a plurality of line patterns in such a manner backward scan print positions relative to the associated forward scan print positions differ from one another while adjusting ink ejection timing. The user visually checks the printed line patterns and selects one with the best straightness. Then, a parameter representing the selected line pattern is entered into the printing apparatus either by directly operating keys on the apparatus or operating a host computer connected to the printing apparatus. The printing apparatus sets optimum ejection timings for the forward scan and the backward scan-based on the parameter entered. After this, when a printing operation is to be done, the print scans are controlled according to the set ejection timings.
When the dot adjust value calculation processing is performed between a plurality of print heads, a plurality of line patterns are printed on one and the same straight line by the print heads. At this time, the line patterns are printed by differentiating their relative ink ejection timings. The user visually checks the printed line patterns and selects one with the least misalignment. Then, a parameter representing the selected line pattern is entered into the printing apparatus either by directly operating keys on the apparatus or operating a host computer connected to the printing apparatus. The printing apparatus sets optimum ejection timings for individual print heads based on the parameter entered. After this, when a printing operation is to be done, the print heads are controlled according to the set ejection timings.
What has been described above is a method that outputs a test pattern for a visual check by the user (referred to as manual dot adjust value calculation processing). This method, however, not only is cumbersome for the user but also is not immune from a possibility of misjudgment and faulty operation. Thus in recent years, a method of automatically performing the dot adjust value calculation processing by using an optical sensor (referred to as automatic dot adjust value calculation processing) has been proposed and put to practical use (e.g., Japanese Patent Application Laid-open No. 11-291470).
The automatic dot adjust value calculation processing disclosed in Japanese Patent Application Laid-open No. 11-291470 will be briefly explained as follows. First, as with the manual dot adjust value calculation processing, a predetermined test pattern is printed by the forward and backward scan of a print head or by a plurality of print heads. Next, a plurality of pattern is printed by shifting other dots (those dots formed, for example, by the backward scan or color print heads) from reference dots (those dots formed, for instance, by the forward scan or a black print head).
The patterns printed in a plurality of different conditions have different area factors (percentage of a dot-occupied area with respect to an overall area of interest) because dots printed under different conditions shift from each other. Based on this fact, the method proposed by Japanese Patent Application Laid-open No. 11-291470 measures an average density of each of the test patterns by an optical sensor, decides that a pattern with the highest average density is the one with the least dot deviation, and then automatically set optimum ejection timing for each scan of each print head. This automatic dot adjust value calculation processing obviates the need for cumbersome setting on the part of the user and eliminates a possibility of misjudgment and erroneous input.
It should be noted, however, that if the print position adjustment can only be done in the automatic dot adjust value calculation mode, the dot print position adjustment may become impossible in the event of a failure for some reason during the automatic dot adjust value calculation processing. Thus, Japanese Patent Application Laid-open No. 11-291470 discloses a construction that provides both the automatic dot adjust value calculation processing and the manual dot adjust value calculation processing and which prompts the user to perform the manual dot adjust value calculation processing only when the automatic dot adjust value calculation processing fails.
As described above, the manual dot adjust value calculation processing requires a cumbersome procedure on the part of the user, who must perform many steps involving outputting test patterns, visually checking them, selecting an optimum condition and entering an associated parameter. Since the determination of a set value is left to the user's judgment, there is a possibility of an erroneous setting. Further, since it takes long to complete the procedure from the test pattern output to the final setting, this manual mode is not advantageous also in terms of time performance. For a novice user the above procedure is particularly cumbersome and is not highly evaluated in terms of customer satisfaction. However, for a user already accustomed to the printing apparatus, since the manual mode allows the user to make his or her own adjustment with a satisfactory precision while the user checks himself, the manual mode may give the user a better impression than the automatic mode.
As to the automatic dot adjust value calculation processing that automatically performs the entire procedure from the test pattern output to the final adjust value judgment, this mode eliminates the input work on the part of the user and the time performance problem and thus, from the standpoint of customer satisfaction, is considered a highly advantageous method. However, the user wishing for a high image quality and who knows how to use the printing apparatus may not like the fact that the automatic mode does not allow the user to check the adjustment procedure as it is processed.
Japanese Patent Application Laid-open No. 11-291470 discloses a method which allows a mode transfer from automatic to manual when the automatic dot adjust value calculation processing fails. Since the automatic dot adjust value calculation processing performs all steps in an open loop and thus is vulnerable to external disturbances, the countermeasure adopted by the cited reference is effective. For example, even when the user feels that something is wrong with the print position control, unless an error is detected during the automatic dot adjust value calculation processing, the print position adjustment continues as is in the automatic mode.
In the present ink jet printing apparatus the dot adjust value calculation processing is one of the preferred means that will contribute to a stable production of quality images. It is, however, difficult to meet all the requirements with a single dot adjust value calculation processing, whether automatic or manual. This is because there is a wide range of users of printing apparatus already in wide use, including those who want to make reliable, highly precise adjustments themselves even if the procedure takes time and many others who want cumbersome steps associated with printer maintenance to be executed automatically.